The correct answer is D. out.
The word “blurt” means to say something suddenly and without thinking. In the context of the sentence, the prisoners are anxious and so they say things without thinking, including pieces of vital information.
The other options are not as likely to be used in this context. The word “on” could be used to mean “to talk about something”, but it is not as likely to be used to mean “to say something suddenly and without thinking”. The word “of” could be used to mean “to be a part of something”, but it is not as likely to be used to mean “to say something suddenly and without thinking”. The word “for” could be used to mean “to be in favor of something”, but it is not as likely to be used to mean “to say something suddenly and without thinking”.